What open source cloud storage system offer an append-only mode for buckets, directories, etc

Question

I'm curious if any cloud storage system could be configured to provide the following workflow :

• Anonymous users may upload messages/files into identifiable locations which we'll call buckets.
• All users should have read access to all messages/files, but no anonymous user should have permissions to modify or delete them.
• Buckets have associated public keys which a moderator uses to authenticate approvals or deletions of uploads.
• Unapproved messages/files are eventually culled by the system to save space.

I suspect the answer might be "Tahoe-LAFS would love for someone to implement append-only mutable files, but nobody has done so yet."

Answer 1

I've surveyed a number of OSS projects in the storage space and not encountered anything that would provide this workflow purely by configuration and without writing code.

While not OSS, the lowest level of Windows Azure storage is actually implemented via an append only mechanism. A video, presentation, and whitepaper can all be found here and the details in the whitepaper would be useful to anyone looking to implement something like this for Tahoe-LAFS or any other OSS cloud storage system.